1. Field of the Invention
The invention relates generally to apparatus for retaining a workpiece in a semiconductor wafer processing system and, more specifically, to an apparatus for controlling heat transfer gas layer uniformity along a bottom surface of the workpiece and eliminating workpiece pop off during dechucking.
2. Description of the Background Art
Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor wafer process chamber. Although electrostatic chucks vary in design, they all are based on the principal of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrodes, respectively. The electrostatic attractive force between the opposite charges pulls the workpiece against the chuck, thereby retaining the workpiece.
In semiconductor wafer processing equipment, electrostatic chucks are used for clamping wafers to a pedestal during processing. For example, electrostatic chucks find use in etching, chemical vapor deposition (CVD), and physical vapor deposition (PVD) applications. One type of electrostatic chuck is fabricated using conventional, flexible printed circuit fabrication techniques with materials available from a flexible circuit board manufacturer such as Rogers Corporation of Chandler, Ariz. More specifically, the electrostatic chuck is comprised of a top layer of dielectric material covering a conductive electrode. Below the electrode is a bottom layer of dielectric material. The top and bottom dielectric layers encapsulate the conductive electrode or electrodes. The entire assembly is then adhered to the pedestal. The top layer of dielectric material forms a support surface upon which the workpiece is retained. When a voltage is applied to the electrode, the wafer is referred back to the same ground reference as the voltage source by a conductive connection to the wafer via an electrical conductor or a plasma formed proximate the wafer. As such, an electrostatic force is established between the wafer being clamped and the electrostatic chuck.
The materials and processes used to process a wafer are extremely temperature sensitive. Should these materials be exposed to excessive temperature fluctuations due to poor heat transfer from the wafer during processing, performance of the wafer processing system may be compromised resulting in wafer damage. The aforementioned pedestal forms both a cathode and a heat sink. To optimally transfer heat between the wafer and chuck, a very large electrostatic force is used in an attempt to cause the greatest amount of wafer surface to physically contact the support surface. However, due to surface roughness of both the wafer and the chuck, small interstitial spaces remain between the chuck and wafer that interfere with optimal heat transfer.
To achieve further cooling of the wafer during processing, an inert gas such as Helium is pumped into the interstitial spaces formed between the wafer and the support surface of the chuck. This gas acts as a thermal transfer medium from the wafer to the chuck that has better heat transfer characteristics than the vacuum it replaces. To further enhance the cooling process, the chuck is typically water-cooled via conduits within the pedestal. This cooling technique is known as backside gas cooling.
The backside gas exerts a pressure on the wafer that pushes it away from the support surface. The electrostatic force exerted by the electrostatic chuck is greater than the force created by the gas pressure; therefore, the wafer remains in position during processing. However, the interaction of these two forces may cause random, localized leakage of the heat transfer gas where the edge of the wafer contacts the electrostatic chuck. Leakage of the gas contributes to an undesirable non-uniform temperature condition across the bottom surface of the wafer.
When processing is completed, the wafer must be quickly and accurately removed from the chamber. A wafer is typically "dechucked" by turning off power to the electrostatic chuck. In this way, the electrostatic force is eliminated and the wafer is no longer clamped to the support surface of the pedestal. Lifting and moving mechanisms (i.e., lift pins from beneath the pedestal and/or a robotic arm) can then engage the wafer to remove it from the chamber.
As the electrostatic force is reduced, the heat transfer gas is evacuated from the interstitial spaces between the wafer and the support surface. A vacuum pump connected to the backside gas inlet port is activated to draw the gas from the process chamber, i.e., the path used to supply the gas is used to remove the gas. Gas pressure is thereby reduced, but not necessarily at the same rate that the electrostatic clamping force is reduced. If the force from the evacuating gas is greater than the clamping force, the wafer will pop off the support surface. Once a wafer has moved from its original position, the lift pins or robotic arm may not properly contact the wafer. As a result, the wafer is pushed off the support surface and into an irretrievable position within the process chamber. Contaminating the wafer in this manner results in a total loss of the wafer as a usable product. The wafer may also be scratched by engaging it in an area that has been processed thereby damaging the delicate circuit patterns formed onto its surface.
Existing wafer processing systems and the chucking apparatus they employ are not adequately equipped to eliminate the leakage of heat transfer gas during wafer processing or the dynamic interaction of unequal forces encountered by a wafer during the dechucking process. There are no provisions for the rapid removal of backside gas. As result, wafer pop off continues to be a condition not l addressed by the prior art.
Therefore, there is a need in the art for an apparatus that provides greater control of heat transfer gas during wafer processing and quickly removes backside gas from beneath a wafer prior to dechucking the wafer.